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TGP3:
Pressure and different substances
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Notes on the effect of substance
The principle that the pressure of a sample does not depend
on the substance, just the concentration of particles and
the temperature, is an important one. It means we can use
the volumes of samples to compare the number of particles,
providing that the samples are at the same temperature and
pressure.
Students should predict that there will be twice as many carbon
dioxide particles, because the volume of the carbon dioxide
sample is twice the volume of the oxygen sample. There are
10 oxygen particles and 21 carbon dioxide particles in the
example, so the prediction is borne out.
Notes on the video and ‘Think for Yourself’ questions
Why does the plunger move?
The liquid water changes to the gas
state because the temperature of the syringe is above boiling
point. The
pressure of the gas water inside the syringe is greater than
the air pressure outside it, so the plunger moves
outwards.
Why does the plunger eventually stop moving?
The pressure inside the syringe decreases
as the volume increases.
The plunger stops moving when the
pressure inside is the same as the air pressure outside.
What would have happened if we had left the syringe to cool
down when the plunger had stopped moving?
Explain your answer.
The plunger would have moved back
in again, because the pressure inside would drop as the temperature
dropped. As the water changes back to the liquid state there
would be fewer and fewer particles of water still in
the gas state, so the pressure would drop more.
What would happen if we had used twice as much water? Explain
your answer.
The plunger would have been pushed
right out! Twice the number of water particles would need
twice the
volume to be at the same pressure.
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Science Enhancement Programme CD-ROM 2005 |
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user
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